Human skin colors are determined by amounts of melanin, carotene and hemoglobin. Among those, melanin plays the most important role. Melanin is produced via enzymatic or non-enzymatic oxidation of tyrosine at cells called melanocytes present at the stratum basale of epidermis, and transferred to keratinocytes forming the epidermis.
Melanin pigment produced at skin melanin cells is a phenolic polymer substance in the form of a complex of black pigment with protein. Such melanin pigments play an important role in inhibiting skin damages caused by ultraviolet rays irradiated from the sun. However, excessive synthesis and accumulation of melanin causes severe aesthetic skin troubles, such as freckles, ephelides and senile plaques, stimulates skin aging or causes skin cancers. It is known that the most important factor in biosynthesis of melanin is functions of tyrosinase present at melanin cells. Thus, most studies about skin whitening and skin cancer prevention through inhibition of excessive production and deposition of melanin are focused on inhibition of tyrosinase activities. Many compounds having tyrosinase-inhibiting activity have been used as materials for whitening cosmetics. However, they are limited in practical application, because they have poor stability to cause decomposition and coloring, generate off-flavors, have unclear effects in bio-level and are problematic in terms of stability.
Therefore, to prevent and overcome such problems, many studies and technological development have been conducted by searching for herb medicines having relatively low toxicity and excellent whitening activity. For example, there are known Korean Patent Publication No. 1993-0010548 (Whitening Cosmetics Containing Broussonetia Extract), Korean Patent No. 10-0535875 (Cosmetic Composition Containing Mixed Plant Extract Having Skin Whitening Effect), Korean Patent No. 10-0654959 (Skin Whitening Cosmetic Composition). However, disclosures of the above patent documents cause problems of skin irritation and toxicity due to the use of herb medicines, and thus are not amenable to skin applications at high concentrations. In addition, when the compositions according to the related art are applied at reduced concentrations, they provide insufficient whitening effects.